Arsenic is a troublesome impurity found in technical grade molybdenum trioxide. It is usually separated from molybdenum by precipitation as magnesium ammonium arsenate from an ammoniacal ammonium molybdate solution and filtration from the solution. Quantitative separation is critical because arsenic must be &lt;8 weight ppm in ammonium dimolybdate. When ammonium dimolybdate (which is obtained by evaporative crystallization from an ammoniacal ammonium molybdate solution) contains arsenic levels higher than 8 weight ppm it must be reprocessed to purify it of arsenic.
Prior methods of purifying ammonium dimolybdate (ADM) of arsenic involve dissolving the ADM in ammonia, and adding magnesium ions to form magnesium ammonium arsenate, which is filtered off. The purified solution is then passed through an ion exchange resin to remove the Mg ions from the molybdate solution. The solution is then evaporated to crystallize ADM. The arsenic is not consistently reduced when these methods are followed. These methods require use of ammonia, and excess magnesium. Therefore, there is an extra step to purify the solution of magnesium. If an ion exchange column is used, it must be regenerated more often because it becomes loaded more quickly because of the added Mg ions.
U.S. Pats. Nos. 3,829,550, 3,848,049, 4,273,745, 4,278,644, 4,298,582,4,320,094,4,525,331, 4,555,386, 4,596,701, 4,601,890, 4,604,267, and 4,604,266 relate to various methods and processes to purify molybdenum.